Under conditions of diminished visibility, FAA decisions regarding aircraft takeoff and landing operations are based on a parameter known as the Runway Visible Range (RVR), or more simply as the “visibility.” RVR is an estimate of how far a pilot can see down a runway and is employed to define operational limits on the use of precision runways. RVR determines what level of equipage is required on both the airfield and any given aircraft to permit takeoff and landing. The RVR is monitored continuously on the ground at each instrumented airfield. In the modern United States implementation, RVR entails the determination of two separate visible ranges, which are VK, or “Koschmieder's visibility,” and VA, or “Allard's visibility.” VK, or Koschmieder's visibility, which is also known as the Meteorological Optical Range (MOR)=3/σ, is a measure of background-scene contrast (such as runway edges) that depends only upon a parameter known as the atmospheric extinction coefficient σ. VA, or Allard's visibility, is the visibility of runway lighting given by ET=(I exp [−σVA])/VA2, where I is the luminous intensity at the source. At any given time, the greater of these two visibilities (VK, VA) is reported as the RVR.
An extinction coefficient measures the degree to which a background scene is “extinguished” by weather conditions, notably fog. Fog is a suspension of water droplets, typically from 1 μm-10 μm in diameter. Long wavelength light, greater than 10 μm (long-wave infrared or LWIR), is transmitted through fog, while short wavelength light comparable to or smaller than the sizes of water droplets (e.g., visible spectrum light in the range of 0.4 μm-0.7 μm) is scattered by fog, obstructing the scene in the background. Thus, certain infrared wavebands, having wavelengths longer than the sizes of the water droplets involved, would logically have a significant fog penetration advantage in comparison to visible light. Mid-wave infrared (3 μm-5 μm) (MWIR) imagers may penetrate fog, depending on the distribution of water droplet sizes, but MWIR sensors have the disadvantage of requiring cryogenic cooling of the detector arrays to maintain operational stability. In the LWIR regime, the intensity of heat emitted from objects within the field of view is maximized. This, together with the fog-penetrating property, makes LWIR the most beneficial range for imaging terrain and obstacles.
Short wave (0.7 μm-2.5 μm) infrared (SWIR) or Near Infrared (NIR) imagers yield little, if any, fog penetration advantage. However, runway lights radiate more heat than light, and this heat is primarily in the SWIR (short wave infrared) range. Therefore, a SWIR detector is useful because it enhances the image of runway lights. In addition, in daytime conditions, solar energy is reduced, compared to visible wavelengths of the runway lights. Therefore sensitivity to the lights is increased at all times, and daytime lights-to-solar contrast is higher. NIR/SWIR also has significant advantages under conditions of haze and smog—an increasingly important consideration in a polluted continental environment.
Traditionally, the extinction coefficient has been monitored through use of a two-point, separated source-and-receiver arrangement known as a “transmissometer.” A transmissometer fundamentally includes a collimated point source transmitter and a single-element receiver with collecting lens, located some distance away. Fog extinction is inferred by measuring the attenuation between source and receiver under obscured conditions, as compared to the throughput on a clear day.
The art of constructing and calibrating transmissometers is a well-established one. Aspects of transmissometer design and performance include:
(a) automatic monitoring of source intensity,
(b) provision of a collimated source beam and narrow field-of-view receiver to minimize reception of forward-scattered light, particularly from multiple scattering in dense fog conditions;
(c) “chopping” and ac-coupling the output if solar energy will otherwise contribute to receiver response;
(d) cleanliness of the sending and receiving optics—freedom from contaminants;
(e) choice of measurement averaging-times, vs patchiness of fog, and wind speeds; and
(f) routine re-calibration on clear days.
As the result of a landmark FAA program, the extinction coefficient at modern United States airports is now derived from a basic, single-point measurement performed by a “forward scattermeter” instead of a transmissometer. The scattermeter has an advantage of a large dynamic range.
For many years, there has been great interest in utilizing infrared cameras (also referred to herein as “imagers” or “sensors”), in both night and daytime conditions, as “see-through-fog” enhancements on transport aircraft and ground-based air traffic control systems. A major, specific goal is to permit approach and landing operations at lower minima than would otherwise be permissible for a given level of aircraft and/or airport equipage. To permit FAA dispatch to a destination and, upon arrival in its vicinity, to permit operations below conventional operating minima for that aircraft and airport equipage, it may be necessary to know for certain that the infrared performance is adequate. A dispatch will usually not be permitted without such assurance. More dramatically, any strategy involving final descent with dependence upon eventual runway acquisition with a missed approach (go-around), should infrared penetration turn out to be inadequate, may not be acceptable. Therefore, the infrared performance at the destination runway must be known in “real time” at time of dispatch and at commencement of final approach.
Infrared Enhanced Vision Systems (EVS) have been in use as supplemental equipment on some aircraft since 2003. A basic EVS system aboard an aircraft includes an externally mounted infrared receiver (also referred to as an imager, sensor, or camera), signal processing circuitry, and a cockpit display. The transceiver may be tuned to a narrow range of infrared wavelengths, or it may employ a multi-waveband system. In the most straightforward configuration, the sensor images are utilized in conjunction with a head-up (HUD) and/or head-down display, in order to extend either manual or automatic landing operations. In more advanced configurations, the sensors can be used to derive independent navigation data for multi-thread integration with other subsystems, including Instrument Landing Systems, digital maps, and augmented Global Positioning Satellite technologies. Although recent performance/cost advances in infrared sensors are dramatic, the locational and seasonal variations in fog structure are such that is not possible to know with certainty how much fog penetration airborne infrared EVS provides, and how often.
Current FAA practice defines three standard visibility categories, relating to a standard aircraft approach glideslope:                1. Category I (nominally 2400 foot visibility at 200 foot altitude)        2. Category II (nominally 1200 foot visibility at 100 foot altitude)        3. Category IIIa (nominally 600 foot visibility at 50 foot altitude).For a given destination, or group of destinations (route structure), it is necessary to know how often the above “certain performance” condition will be satisfied by the infrared sensors. However, unlike the above, this is now a statistical consideration. For example, for a given destination and season, if it could be established, for 90% of Category IIIa visibility conditions, that infrared EVS would provide runway acquisition at a Category I decision height, then the economic value would justify the required investment in the technology.        